Greetings,
I am trying to connect my Mac to a broadcast monitor (BVM1900A Sony) it is an old monitor I got second hand and I have never set up a monitor straight to a computer before. I have a VGA to BNC RGB cable.
I connected the mac that is set up with dual monitors with the VGA to BNC cable, the unit powers up, but I get no indication that the BNC cable is transferring any information. Only a grey screen.
any ideas?

MAC's require a video card that will provide a video output, not a computer output.
There are countless manufacturers like AJA, Blackmagic, Matrox, and MOTU that make cards and boxes that do this (and there are plenty of others that make scan converters like Folsom, Kramer and Communication Specialities) that will allow you to do this as well.

But you say "but I don't want to spend any money - cant I just get a cable to do this" -

NO - you lose ! Tough noogies. Crack open that wallet, and start spending some money.

Thanks!
great, I suppose I will crack open the old wallet for this. to monitor HD video, I need a digital monitor though right? Am I just being silly trying to use this old 1980s big box monitor? Monitors for HD are specially made LCD screen monitors, not big broadcast box monitors, am I right?